The invention relates to an NTC temperature sensor, particularly for use in the exhaust gas system of internal combusion engines, and also a process for producing NTC temperature sensing elements for the NTC temperature sensor.
It is generally known to use NTC temperature sensors having a temperature sensing element disposed in a housing to measure comparatively high temperatures such as prevail, for example, in the exhaust gases of internal combustion engines. Electron-conducting shaped oxide bodies and also, in particular, ion-conducting solid electrolyte shaped bodies, the latter based on oxides of tetravalent metals such as, in particular, ZrO.sub.2, CeO.sub.2, HfO.sub.2 and ThO.sub.2 having a certain content of divalent alkaline earth oxides and/or, preferably, trivalent oxides of the rare earths are suitable for producing the temperature sensing elements. Characteristic of such solid electrolytes is the fact that their electrical resistance decreases rapidly at temperatures from about 300.degree. to 1200.degree. C.
In the simplest manner, NTC temperature sensing elements have disc-type, pill-type or bead-type shaped resistance bodies having electrodes of a conductive material disposed on their sides situated opposite each other. To protect the shaped resistance bodies, it is furthermore known to house them in gas-tight housings or to coat them with a layer of glass. NTC temperature sensing elements of the type describe are known, for example, from German Auslegeschrift 2,429,866, German Patent Specifications 2,321,715 and 2,333,189 and also EP-A-0,056,752.
To determine the .lambda.-value of gas mixtures, it is furthermore known to use planar exhaust gas sensors which can be produced in a particularly cheap manner by ceramic-foil and screen-printing technology. In this connection, reference is made, for example, to EP-A-0,188,900 and 0,142,993 and also German Offenlegungsschrift 3,017,947 and 3,543,759.
Disadvantages of the known sensing elements mentioned above is the fact that they age too rapidly during use, that their response times are too long and that their method of production is too expensive and/or their dimensions are too large. A particular disadvantage is the dependence of the characteristic curves of the resistance of such NTC materials on the composition of the gas, in particular in reducing gases or gas mixtures which can bring about oxygen deficiencies in the oxidic NTC resistance materials and thus alter their resistance values. The unavoidable diffusion of oxygen through highly refractory metal housings or through glass envelopes, in particular, causes undesirable changes in the resistance values.